Hydrodesulfurization process for producing a heavy asphaltic fuel oil

ABSTRACT

ASPHALT-CONTAINING OILS ARE HYDRODESULFURIZED TO BELOW A REFRACTORY SULFUR LEVEL WITHOUT EXCESSIVELY CONVERTING THE ASPHALT PRESENT BY EMPLOYING A TWO-STAGE HYDRODESULFURIZATION SYSTEM WITH INTERSTAGE HYDROGEN SULFIDE AND LIGHT OIL REMOVAL, WHEREIN THE AVERAGE HYDROGEN PARTIAL PRESSURE IN EACH HYDROESULFURIZATION STAGE IS MAINTAINED SUFFICIENTLY HIGH THAT THE AVERAGE REACTION RATE CONSTANT IS IMPROVED BY A TWO-STAGE OPERATION, BUT SUFFICIENTLY LOW SO THAT NOT LESS THAN ABOUT 50 PERCENT BY WEIGHT OF THE FEED IS RECOVERED AS A PRODUCT BOILING ABOVE THE I.B.P. OF THE FEED.

Nov. 21, 1973 D MCK|NNEY ET AL I-IYDRODESULFURIZATION PROCESS FORPRODUCING A HEAVY ASPHALTIC FUEL OIL Filed Dec. 8, 1971 5 Sheets-Sheet 1Nm. 4 o:

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NOV. 27, 1973 D, MCK|NNEY ET AL. 3,775,305

HvnnonlcsplmlumZATTON PRGCESS FOR I'RODUGING A HEAVY ASPHAIRTC FUEL OILJFiled Dec.r S, 1971 5 sheets-sheer z FIG. 2

250- STAGE 2 Z Z T O l O l m I *a r 200- i '50- i STAGE T i O y O //O/(E E ,o E 5 50- A 09 l 1 HY0R0GEN RART|AL PRESSURE, RSI

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770 AVERAGE RECTOR` TEMPERATURE: F.

FIG. 3

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Nov. 27, 1973 1 D. McKlNNEY ET AL 3,775,305 v HYDRODESULFURIZATIONPROCESS FOR PRODUCING A HEAVY-ASPHALTIC FUEL OIL Filed Deo. 8, 1971 5Sheets-Sheet 4 IO O FIG. 5

VOL. /o OF COMPONENT IN OVERHEAD l foo 20o 30o 40o so@ @OO 700 800 ENDPOINT, F.

Y S /r 326 y 330 l I 34o 343 352 FI G. 7 342 356 l NOV. 27, 1973 J, D,McKlNNEY ET AL 3,771,305

HYDRODESULFURIZATION PROCESS FOR PRODUCING A HEAVY ASPHALTIC FUEL OILFiled Dec. E, 1971 5 Sheets-Sheet 5 United States Patent O 3,775,305HYDRODESULFURIZATION PROCESS FOR PRO- DUCING A HEAVY ASPHALTIC FUEL OILJoel D. McKinney and `lohn A. Paraskos, Pittsburgh, Pa.,

assignors to Gulf Research & Development Company, K Pittsburgh, Pa.

Filed Dec. 8, 1971, Ser. No. 206,083

Int. Cl. Cg 23/02 U.S. Cl. 208-210 n Claims ABSTRACT oF THE 4DISCLOSUREThis invention relates toa process for the production of crude orresidual oils having an especially low sulfur content. Moreparticularly, this invention relates to a hydrodesulfurization processfor producing an asphaltic fuel oil having a low sulfur content.

Various processes have been proposed for the desulfurization ofpetroleum hydrocarbons, and such processes have been highly effective inthe removal of sulfur from the relatively lighter distillate fractionsthat are obtained from the distillation of crude oil. However, crude oiland reduced crude oil fractions contain a heavy, residue fractionYboiling above 1040 F. commonly referred to as asphalt, which renders theoil much more diicult to desulfurize. Asphalt is a generally low-gradematerial that has been isolated from the crude oil and utilized in roadconstruction. In addition, it has been proposed to treat the asphalticfraction of crude oil in some manner so as to upgrade this material intoa more valuable product such as a fuel oil. However, asphalt, whenisolated, is highly viscous and is diicult to process over a catalyst.Moreover, it consists of large molecules of fused, aromatic rings andcontains the greatest amount of the total sulfur content of the fullrange crude on a relative basis. Afurther problem is that some of thesulfur content of the asphalt is tied up in the interior of the` largemolecules thereof, rendering the asphalt especially'diicult todesulfurize. In addition, asphalt contains metals, principally nickeland vanadium, which readily deposit on desulfurization catalysts tendingto deactivate the catalyst. The cumulative effect of such factors isthat much more severe conditions in the form of higher temperatures andpressures are required as well as a different type of catalyst in'orderto remove the sulfur-from heavy, asphaltic oils as compared withdesulfurization of a distillate oil.

VSulfur is a major lcontributor to vair pollution. Accordingly, certainmunicipalities, both local and foreign, have placed an upper limit onthe sulfur content of fuel oils. Inthe past, such locales `haveplaced aone percent by weight sulfur limit on heavy fuel oils, but the trend hasbeen to a lower maximum sulfur content, such as 0.5 weight percent. Suchlimitations have provided a major impediment to the use of heavyasphaltic hydrocarbon oil fractions as fuel oil, since an asphaltic fueloil having an 0.5 percent sulfur content is not easily obtained fromsome high sulfur crudes merely by passing the crude oil over a`desulfurization catalyst.

For reasons previously unappreciated, the sulfur content of a particularasphaltic crude or reduced oil could be reduced to one percent sulfur ina hydrodesulfurization zone, but if the sulfur level was to be reducedbelow one percent, the desulfurization temperature had to be increasedto a point at which excessive hydrocracking to lower boiling materialsresulted. Thus, while it had been relatively easy to reduce the sulfurlevel of such residual oils from 4 to l percent by weight of sulfurwithout incurring excessive cracking of the feed, it is substantiallymore diicult for a majority of typical feeds to remove more than aboutpercent of the sulfur without excessive hydrocracking. Excessivehydrocracking is undesirable since it unnecessarily consumes hydrogenand produces undesired light products and coke on the desulfurizationcatalyst. Furthermore, the nature of the resulting product can bealtered to the extent that it is not a heavy fuel oil whereas heavy oilsgenerally have a higher heating value than lighter oils.

Alternatives to attempted in-depth desulfurization of such asphalticfractions have included blending of the asphaltic hydrocarbon oil withessentially sulfur-free middle distillate oils in order to provide a lowsulfur-containing fuel oil. Other proposals have involved hydroreliningasphaltenic oils, under conditions wherein the asphaltic material ishydrocracked to extinction to produce lower boiling hydrocarbonproducts.

It would be highly desirable to provide a process for desulfurizingasphalt-containing hydrocarbon oils so as to render them suitable foruse as heavy fuel oils without the need for either blending suchmaterials with lighter oils or converting the asphalt content of suchoils to lower-boiling products of a completely different nature whichwould prematurely deactivate the catalyst.

It has now been found that a particular heavy fuel oil containing anasphalt fraction may be produced having a sulfur content below onepercent by weight of means of the present invention which comprises aprocess which involves passing an asphalt-containing or asphaltic,hydrocarbon oil, such as a crude oil or residual oil (a reduced crude)containing more than about one percent sulfur to a firsthydrodesulfurization zone under an average hydrogen partial pressure,Pav, withdrawing a first effluent from the rst hydrodesulfurization zonehaving a reduced sulfur content relative to the feedstock, said firsteflluent comprising hydrogen sulde, a light gas fraction, an oilfraction containing aromatics and saturates, and a higher boilingasphaltic fraction. The oil fraction is referred to as light oil tocontrast it to the higher boiling asphaltic fraction. The hydrogensulfide, the light gas fraction, and a low boiling portion of the lightoil fraction are separated from the first ellluent, and the remainingportion of the rst effluent comprising the asphaltic fraction and ahigher boiling portion of the oil fraction is passed to a secondhydrosulfurization zone and is desulfurized under an average hydrogenpartial pressure, Pzav. Each hydrodesulfurization zone is provided witha hydrodesulfurization catalyst that is disposed on a non-crackingsupport. A second eflluent is withdrawn from the secondhydrodesulfurization Zone which provides a heavy, asphaltic fuel oilcontaining less than one percent by weight sulfur. The average hydrogenpartial pressure, Pav, is maintained above the hydrogen partial pressurevalue at which the reaction rate constant for the lirst zone 4would havethe same value as the reaction rate constant for the second zone, butsufficiently low that not less than 40 percent by weight of said secondefuent boils above the I.B.P. of the feed to said rsthydrodesulfurization zone.

Pv is defined as follows:

PIBV PZIV wherein P18, is the average hydrogen partial pressure in thefirst hydrodesulfurization zone; and

P2M, is the average hydrogen partial pressure in the secondhydrodesulfurization zone.

Surprisingly, it has been discovered that by maintaining the averagehydrogen partial pressure in each of the hydrodesulfurization zones of atwo-stage desulfurization process above a predetermined point, as willbe hereinafter described, the highly refractory sulfur present in theasphaltic portion of the oil may be effectively removed at adesulfurization reaction rate that is unobtainable when employing asingle stage operation.

The removal of a controlled portion of the normally liquid, light oilfraction, which ,fraction comprises both aromatics and saturates havinga boiling point below that of the asphalt fraction contained in theoriginal crude or reduced crude, that is passed along with the asphalticfraction to the second hydrodesulfurization zone, results in thereduction of the sulfur content of the resulting heavy crude oil moreeasily and with less catalyst to below one percent, for example, tobelow 0.5 and 0.3 percent by weight sulfur, than is possible in a singlereaction zone. The removal of the light oil fraction inherently removessubstantially all the hydrogen sulfide and light hydrocarbons producedin the first stage so that this material does not enter the secondstage.

One modification of the present invention involves a process whichcomprises passing an asphaltic, hydrocarbon Ifeed containing more thanabout one percent sulfur to a hydrosulfurization zone in the presence ofhydrogen and Ia light oil fraction comprising aromatics and saturateshaving a boiling point below the asphaltic portion of the feedstock tosaid hydrodesulfurization zone, and controling the amount of said lightoil, and thus the amount of aromatics and saturates that are passed tothe hydrodesulfurization zone to increase the desulfurization rate andpermit the recovery of a heavy, asphaltic hydrocarbon fuel containingless than about one percent sulfur.

Thus, the present invention provides an asphalt-containing heavy fueloil, which is practically sulfur free, without the need for blending theasphaltic fuel 'oil with a sulfur-free middle distillate oil in order toobtained an oil having a low sulfur content, although such blending isnot precluded in accordance with the present invention.

A still further modification of the present invention relates to aprocess .for controlling the hydrodesulfurization of an asphaltic, heavyoil, which process comprises passing crude oil or residual oil, forexample, which oil contains an asphaltic fraction, through ahydrodesulfurization zone in the presence of hydrogen, the oilincreasing in aromatic hydrocarbon content as it passes through thehydrodesulfurization zone, controlling the aromatic content of the oilas the oil passed through said hydrodesulfurization zone, and thenwithdrawing the oil from said hydrodesulfurization zone at substantiallythe point 'at which the aromatic content of the oil no longer increases.

The term asphalt or asphaltic as employed in the present specificationis intended to include the resins and asphaltenes present in crude oil.Asphalt can constitute approximately 5 to 30 percent by volume or moreof crude oil and has an initial boiling point of about l040 F. It isobtained in refineries by a propane deasphaltng process (solventextraction), or from the residues obtained from distillation.Asphaltenes are highly aromatic and consist of large molecules of fusedaromatic rings and normally contain the greatest sulfur concentration ofany constituent of the full range crude. Unlike other crude fractions,asphalt also contains metals, principally nickel and vanadium. Thus, theasphaltenes and resins may be distinguished from the remainder of thecrude oil, which material comprises saturates and aromatics, by virtueof the solubility of these aromatics and saturates in propane and theinsolubility of the asphaltenes and resins in propane. n

The propane-soluble aromatics include benzenes, naphthalenes,thiophenes, benzothiophene, and dibenzothiophenes las the predominantmolecular species, while the saturates include the non-aromatic,propane-soluble species, such as the naphthenes, e.g., cyclohexanes,andthe parafiins, e.g., dodecane, and sulfur-containing compounds, suchas s-butyl mercaptan. Thus, the material commonly referred to as asphaltcomprises the residue of a propane extraction. On the other hand, resinsand asphaltenes are themselves separable by a pentane extraction, byvirtue of the fact that asphaltenes are insoluble in pentane, while bothresins and oils are soluble in pentane.

It has been proposed to subject crude oil or a reduced crude containingthe asphaltene fraction to a hydrodesufurization reaction in order toreduce the sulfur content of the crude. The crude oil or reduced crudeis passed over Group VI and Group VIII metals on a non-cracking support,such as alumina, in the presence of hydrogen and the sulfur level isrelatively easily reduced from about 4 to about 1 percent by weight,i.e., a 75 percent sulfur removal. However, after about 75 percentsulfur removal is effected with a particular feed, such as a Kuwaitcrude for example, the sulfur content of the feed suddenly becomes quiterefractory and a great deal of hydrocracking is experienced in order toaccomplish ad` ditional sulfur removal thereby consuming a great deal ofhydrogen and changing the nature of the product. Thus, the point atwhich the sulfur remaining in the crude oil becomes refractory may varywith the nature of the particular type of crude oil. This point may beeasily determined experimentally. As previously mentioned, theasphaltenes consist of large molecules of fused aromatic rings andcontain sulfur in the interior of the large molecules, thereby renderingthe sulfur extremely diicult to remove. In addition, the asphaltcontains all of the metals, such as nickel and vanadium that are presentin the crude, and these metals readily deposit on the catalyst tendingto deactivate the catalyst and reduce its effectiveness. For thesereasons, more severe conditions in the form of higher temperatures andpressures are required to remove more than 75 percent of the sulfur fromthe asphaltic oil and such severe conditions result in hydrocracking,i.e., a severing of the carbon-carbon bonds of the asphaltene moleculeresulting in lower molecular weight materials, rather thandesulfurization by splitting carbon-sulfur bonds.

In order to more fully understand the process of the present invention,reference is made to the drawings wherein:

FIG. 1 is a ow diagram illustrating the desulfurization of an asphaltic,reduced crude in two stages;

FIG. 2 graphically illustrates the effect of hydrogen partial pressurein each desulfurization stage upon the desulfurization reaction rateconstant;

FIG. 3 illustrates graphically thepercent yield of materials boilingabove the initial boiling point of the feed to a desulfurization reactoras the average reactor temperature increases;

FIG. 4 graphically illustrates the change in concentration of thearomatic, saturate, asphaltene and resin fractions of anasphalt-containing reduced crude as the degree of desulfurizationincreases;

FIG. 5 graphically illustrates volume percentages of aromatics andsaturates that are removed at various interstage flash points; i

FIGS. 6 to 10 are diagrammatic schemes for obtaining a low sulfur, lightaromatic-rich fraction from one portion of the feedstock to solublizeand reduce the viscosity of the solution of asphaltenes and resinspresent in the residual portion of the feed prior to desulfurization ofthe asphaltenes and resins.

FIG. 1l graphically illustrates the reaction rate advantage of atwo-stage hydrodesulfurization system as contrasted with a single stageoperation.

Referring to FIG. 1, a reduced crude such as a 50 percent reduced Kuwaitcrude which contains the entire asphalt content of the full crude andtherefore also contains all of the nickel and `vanadium and mostrefractory sulfurvcontent of the full crude is charged to the processthrough line and is pumped through line 14, preheater 16, line 18,solids filter and line 22 to drum 24. From drum 24 the liquid oil chargeis passed through line 26 to feed pump 30.

Liquid from pump 30 is admixed with hydrogen from line 32 and passedthrough line 34, valve 36, line 38 and furnace 40. Liquid flow valve 36is disposed in a nonfully preheated liquid hydrocarbon line. Recycledhydrogen along with make-up hydrogen (if desired) are introduced intothe liquid charge to the reactor prior to the preheating thereof.Recycled hydrogen is passed through line 42 and valve 44, while makeuphydrogen may be charged through line 46, compressor 48 and valve 50. Therecycled hydrogen and any make-up hydrogen are introduced to therelatively cool liquid charge through line 32.

A preheated mixture of liquid charge and hydrogen in line 54 may bepassed through a guard reactor (not shown), if desired. An eluent streamfrom the guard reactor is charged to the main reactor 60 containingcatalyst beds 62, 64 and 66. This stream may have a 650 F.| boilingrange, for example.

The hydrodesulfurization catalyst employed in the process of theIpresent invention is conventional and comprises, for example, Group VIand Group VIII metals on a non-cracking support. Thus, the catalyst maycomprise nickel-cobalt-molybdenum or cobalt-molybdenum on an aluminasupport. The alumina may be stabilized with 1 to 5 percent by weight ofsilica. The preferred catalyst is'` a nickel-cobalt-molybdenum lonalumina containing less than 1 percent silica which catalyst may or maynot be sulided. Magnesia is also a non-cracking support. An especiallypreferred catalyst comprises a particulate catalyst comprising particlesbetween about lo and 1/40 inch in diameter, such as described in U.S.Pat. 3,562,800 to Carlson et al., which patent is hereby incorporated byreference. The same or a different hydrodesulfurization catalyst can beemployed in each stage.

`An essential feature of the present invention is that the catalyst isprovided on a non-cracking support, since the process of the presentinvention is essentially a noncracking process in that vary littlematerial is produced having a boiling point below the initial boilingpoint of the feed. However, high 'boiling material in the feed may becracked to produce lower boiling products still in the boiling range ofthe feed. Thus, whereas prior processes have employed techniquesinvolving, for example, high silica-containing catalysts, e.g. 10percent or more silica, to hydrocrack the` asphaltenes in the feedstock,the process of the present invention involves mainly the severance ofcarbon-sulfur bonds of the asphaltenes and resins inorder to desulfun'zethe diicultly desulfurizable asphalt, rather than cracking carbon-carbonbonds, which results in lower molecular weight materials. Of course,sulfur is removed from oils during the present process, but this type ofsulfur removal is relatively easy to accomplish. In other words, thecracking of carbon-carbon bonds as in` prior processes employingcracking produces a material other than a heavy fuel oil and is outsidethe scope of the present invention. Thus, a relatively minor amount ofmaterial is produced having a boiling point below the initial boilingpoint of the feed to the hydrodesulfurization unit.

As previously mentioned, hydrogen is introduced along with the feedstockby means of line 32. Conventional reaction conditions in thehydrodesulfurization reactor are employed, for example, a hydrogenpartial pressure of 1000 to 5000 pounds per square inch, preferably 1000to 3000 pounds per square inch, is employed. A hydrogen partial pressureof 1500 to 2500 pounds per square inch is especially preferred. The gascirculation rate may be between about 200 and 20,000 standard cubic feetper barrel, generally, or preferably about 3000 to 10,000 standard cubicfeet per barrel of feed, and preferably containing 85 percent or more ofhydrogen. The mol ratio of hydrogen to oil may be between about 8:1 and80:1.

As previously indicated, an essential feature of the present inventionis the employment of a predetermined, average hydrogen partial pressurein each of the two hydrodesulfurization zone which will permit thedesulfurization of the asphaltic oil of the present invention to thenecessary extent without undue conversion of the asphaltenes and resins.It is necessary to employ a sufliciently high hydrogen partial pressure,particularly in the second hydrodesulfurization stage where the removalof the highly refractory, asphaltic sulfur takes place, in order toprovide hydrogen to the reactive surface of the asphaltene molecule. Itis the chemical activity as expressed in the partial pressure ofhydrogen, rather than total reactor pressure, which determineshydrodesulfurization activity.

Referring now to FIG. 2, two curves are shown which relate hydrogenpartial pressure to the hydrodesulfurization reaction constant k,defined as 1 1 1e (T-f) LHsv 'i wherein:

Sp=pounds of sulfur per pound of oil in the product; Sf=pounds of sulfurper pound of oil in the feed; and LHSV=volume of oil per hour per volumeof catalyst.

sure in the rst stage should be above about 1000 to 1400 p.s.i. for theparticular fed and catalyst employed, there is little advantage inemploying hydrogen partial pressures above 1400 p.s.i.

On the other hand, the solid line curve of FIG. 2 shows the effect ofhydrogen partial pressure upon the reaction rate constant in a secondstage following a 650 F. ash (corresponding to atmospheric pressure) ofhydrogen sulfide, light gases and a light fraction of the aromatics andsaturates, At hydrogen partial pressures in the second stage below 1400p.s.i. (the point where the curves intersect), the reaction rateconstant is generally lower in the second stage as compared with thereaction rate constant for the rst stage. Moreover, as the hydrogenpartial pressure is increased above 1400 p.s.i., the reaction rateconstant in the second stage very rapidly outranges the reaction rateconstant in the first stage. Accordingly, there is an unexpectedadvantage of maintaining the average hydrogen partial pressure,

above the hydrogen partial pressure value at the point of intersectionof the two curves, i.e., the point at which the reaction rate constantfor the first stage has essentially the same value as the reaction rateconstant for the second stage.

While FIG. 2 illustrates the advantage of employing increasing hydrogenpartial pressures, a point is reached, in one example, at a hydrogenpartial pressure of 2200 p.s.i. where the hydrogen partial pressurebecomes significantly high that excessive hydrocracking occurs, sincehigh hydrogen partial pressures induce hydrocracking. Accordingly, theupper hydrogen partial pressure must not exceed 2200 p.s.i. for thesystem illustrated in FIG. 2. Otherwise, the process becomes a crackingprocess, rather than a non-cracking process, and with all the attendantdisadvantages thereof. Accordingly, the average hydrogen partialpressure of the first and second hydrodesulfurization stages must besufficiently low that not less than 40 percent by weight of the effluentfrom the second stage boils above the I.B.P. of the feed to the firsthydrodesulfurization stage.

Thus, the process of the present invention provides a yield of not lessthan 40 or 50 and up to 80 and 90 percent by weight of material having aboiling point greater than the initial boiling point (I.B.P.) of thefeed to the first desulfurization reactor. Very little, if any,hydrocracking occurs in accordance with the present invention and thehydrogen consumption will be generally in the range of only 150 to 1500,and preferably in the range of 300 to 1000 standard cubic feet perbarrel of feed. The feed to the hydrodesulfurization reactor can have anI.B.P. of not less than 375 F., and will preferably have an I.B.P. of atleast 620 or 650 F. Thus, the amount of material obtained from thesecond hydrodesulfurization stage whose boiling point is lower than 375F., 620 F., or 650 F. will not exceed 50 or 60 percent by weight,generally or preferably 10 or 20 weight percent. A feed having an I.B.P.greater than 650 F., e.g. vacuum tower bottoms having an I.B.P. in therange of 750 to 900 F. or more, may be employed in the process of thepresent invention. In that event, the amount of material obtained fromthe second hydrodesulfurization zone whose boiling point is below 650 F.will not exceed l0 or 20 weight percent.

Likewise, the fuel oil product from each hydrodesulfurization unit has atotal asphaltenes plus resins content of at least l or 20 and can have30 or 40 up to 80 percent by weight of that present in the feed to thefirst hydrodesulfurization unit. The fuel oil product from the secondhydrodesulfurization unit has a preferred total content of resins plusasphaltenes of at least 40-50 or 70 up to 90 percent by weight of thatpresent in the feed to second hydrodesulfurization zone. This is afurther indication that the resins, and particularly asphaltenes may bedesulfurized without their complete destruction as was previouslyproposed.

The point of intersection of the stage I and stage II curves of FIG. 2will vary depending upon the catalyst and the feedstock that undergoeshydrodesulfurization. With some catalysts and/ or feedstocks, the pointof intersection may be as low as 1000 p.s.i., so that effective twostageoperation can occur between 1000 p.s.i. and 2200 p.s.i. Likewise, thepoint at which excessive hydrocracking occurs may vary both below andabove 2200 p.s.i., e.g. 2500, 3000 or 3500 p.s.i.

The data for FIG. 2 was obtained by charging a 4.1 weight percentsulfur-containing Kuwait reduced crude to a first stage to reduce thesulfur content to one percent, and the eflluent is flashed at 650 F.+based on atmospheric pressure. The sulfur content is reduced to 0.5weight percent in the second stage. The catalyst in each stage is anickel-cobalt-molybdenum on alumina catalyst.

In certain instances, it is advantageous to employ a higher hydrogenpartial pressure in the second stage than that which is employed in thefirst stage. However, the hydrogen partial pressure in the first stagemay be substantially the same as that employed in the second stage, ifdesired.

Hydrodesulfurization reactor temperatures may range between about 650and about 900 F., and preferably between about 680 and 800 F.

Referring again to FIG. 1, each succeeding catalyst bed 62, 64 and 66,may have a larger volume than the bed just prior to it. If desired,.there may be 4to 6 catalyst beds in the reactor and, if desired,eachvreactor bed may have 25 percent, 50 percent, 100 percent Ior morecatalyst than the bed just prior'to it.

As previously mentioned, because there is no guard chamber line 54 willlead directly to the reactor. The eflluent in line S4 is joined by ahydrogen stream from line 67, valve 69 line 70 and valve 72 so that ahydrocarbon and hydrogen stream is charged to the top of the reactorthrough line 74. As previously mentioned, the reactor stream is passedthrough catalyst bed 62 and due to the exothermic nature of thehydrodesulfurization reaction, it becomes heated in passagetherethrough. Temperatures between the various catalyst beds may becontrolled by employing a hydrogen quench which may be introduced bymeans of line 76, valve 78, sparger 80, and line 82, valve 84 andsparger 86. Finally, the reaction mixture passes through catalyst bed 66and then leaves the reactor with, for example, about a one percent byweight sulfur content.

Referring now to FIG. 3, it is seen that as an asphaltic hydrocarboncrude oil residue containing 4 percent sulfur is passed through ahydrodesulfurization zone, such as reactor 60 in FIG. 1, to produce a 1percent sulfur-containing 660 F.+ product, well over 80 percent of theproduct boils at 660 F. or above (the initial boiling point of the feed)as the average reactor temperature is increased to compensate forcatalyst aging. However, when the reaction temperature is raised to 800F., excessive hydrocracking to lower boiling materials results. Thiscauses a drop in the curve as to the materials boiling below the initialboiling point of the feed. At this point excessive hydrocracking occursin order to produce a 1 percent sulfur-containing 660 F.+ product.

As will be hereinafter demonstrated, the process of the presentinvention permits the eflicient removal of greater than 75 percentsulfur from an asphaltic oil and overcomes the refractory nature of suchoils.

Reference is now made, once again to FIG. l. Next, the eflluent in line88 is passed to a high pressure flash chamber 90 wherein lighthydrocarbon gases, hydrogen sulfide, hydrogen and a controlled portionof the relatively highly desulfurized saturates and aromatics areremoved by means of line 92. The operation of this interstage flashchamber 90 constitutes a critical feature of the present invention. Bycarefully selecting the flash temperature at the process pressure, theproper amount of light oil, which is to be removed from the feedstock tothe second hydrodesulfurization feed stream is determined. The reasonfor the removal of an amount of light oil will be demonstrated byreference to FIG. 4.

Referring now to FIG. 4, the composition of a 650 F.+ hydrocarbon oilcontaining various proportions of aromatics, saturates, resins andasphaltenes is presented,

as the oil is passed through a hydrodesulfurization reactor, such asreactor 60 of FIG. 1. As shown in FIG. 4, the resins and asphaltenescontent of the feed steadily decreases with increasing sulfur removaldue to the severing of carbon-sulfur bonds thereby breaking offmolecular fragments. The accumulation of these molecular fragments isreflected in the build-up of lower molecular saturates and aromatics andparticularly aromatics. This increase in aromatics content in the liquidis beneficial because the aromatics constitute a solvent for the highlyviscous resins and asphaltenes, whereas the resins and asphaltenes arenot solvated by saturates. The desulfurization of each fractioncontinues until about 75 percent sulfur removal, and at that point theresins and aromatics curves reach a plateau which indicates no furthercracking of fragments therefrom. At the same time, the total aromaticsand saturates content does not increase further, but an increase insaturates level is accompanied by a decrease in aromatics level. Thisindicates that at 75 percent sulfur removal the aromatics tend to becomesaturated, which represents not only a fruitless consumption ofhydrogen, but deprives the remaining resins and asphaltenes of aromaticsolvent, while merely increasing the amount of saturates, Which is amere dispersent.

At the 75 percent sulfur-removal level, the sulfur becomes refractory tofurther desulfurization so that further desulfurization is accompaniedby a loss in aromatics and a sharp increase in saturates. Both of thesefactors are detrimental in regard to removing further sulfur andovercoming the refractory nature of the oil at this point, since at the75 percent sulfur-removal level most of the unremoved sulfur isconcentrated in the resins and asphaltenes. Thus, a loss of aromaticsdeprives the viscous resins and asphaltenes of some solvation, while theformation of saturates imparts an excessive dispersion to the systemtending to excessively dilute the remaining sulfur and thereby lower thereaction rate.

Thus, while operating the first hydrodesulfurization reactor, thearomatic content of the oil can be measured as it passes through thereactor, and the oil should be Withdrawn from the reactor when thearomatic content of the oil is no longer increased. This was thesituation at 75 percent desulfurization in FIG. 3. The aromatic contentcan increase 25 to 40 percent by Weight or more as it passes'through thereactor. Even a small increase in aromatics concentration is beneficial,e.g. 2 to 5 or 10 percent by weight. Of course, the aromaticsconcentration should not increase to such a great extent that it undulydilutes the sulfur Wh'ich is to be removed.

Without attempting to limit the present invention to any particulartheory or mechanisms, it is believed that up to the 75 percent sulfurremoval there is a removal of fringe sulfur from the complexheterocyclic ring structure of the asphaltene molecule accompanied by anin situ production of aromatic hydrocarbons mainly due to a severing ofthe carbon-sulfur bonds on the fringe aromatic rings. This produciton ofaromatics is important, since asphaltene molecules have a tendency toform colloidal crystallites or aggregates, if the molecules are poorlysolubilized. Thus, up to the 75 percent sulfur removal level, there issullicient in situ production of aromatics to solubilize the asphalteneparticle and permit contact of the internal, heterocyclic sulfur withthe hydrogen and catalyst necessary to desulfurize the asphaltene.However, at the 75 percent desulfurization level, the ratio of aromaticsto saturates is sufficiently small that there is insufficient solventand too much diluent to effect the removal of the refractory sulfur.

For this reason, it is necessary to control the aromatic content'of theasphaltic feed to the second hydrodesulfurization zone. This may beaccomplished by flashing or otherwise separating, e.g. by distillationor flashing followed by partial condensation of flashed hydrocarbon andrecycle, a controlled portion of the normally liquid effluent oil fromthe first desulfurization zone to provide the proper aromatics contentto the stream.

Thus, it is essential to include sullicient aromatics in the feed to thesecond desulfurization reactor to solubilize the resins and asphaltenesin the oil and to deagglomerate any asphaltene aggregate which may beformed. At any given flash temperature, the removal of a particularquantity of aromatic hydrocarbons normallyremoves an evengreaterrquantity of saturated hydrocarbons. With certain crudes theflash may remove more aromatics than saturates, in which case thebenefit of the flash will be derived from removal of excess dispersent.A compromise must ,be reached wherein sufficient aromatics `are chargedalong with the feed to the second hydrodesulfurization stage so that theresins and asphaltenes are adequately solvated in order to permit properdesulfurization of the resins and asphaltenes, without the quantity ofaromatics being so high that the total aromatics and saturates fed tothe second stage will excessively dilute and disperse the sulfur to beremoved and thereby reduce the reaction rate. The volume 10 of aromaticsand saturates, respectively, which may be removed at a given flash pointis shown in FIG. 4.

Referring now to the solid lines of FIG. 5, an analysis of the saturateand aromatic content of the first sulfur removal stage eilluent is shownfor various flash points when measured at atmospheric pressure. Thus, ifthe interstage flash point is 500 F., all of the aromatics boiling above500 F. will enter the second stage and will be available for solvatingthe resins and asphaltenes. However, at 500 F., the stream comprisesabout 81 percent by volume saturates and about 19 percent by volumearomatics. This amount of aromatics together with the even greateramount of saturates which necessarily accompany it, might excessivelydilute the asphaltic compounds and thereby lower the reaction rate. Onthe other hand, if the interstage flash point is as high as 800 F., theoil fed to the second stage will have a higher ratio of aromatics tosaturates which is desirable in order to accomplish high solvation withminimum diueunt. However, at the 800 F. flash point, the total amount ofaromatics which enter the second stage may not be great enough todisolve the resins and asphaltenes and to sufliciently lower theirviscosity to permit desulfurization. Accordingly, a proper balance ofaromatics to saturates must be employed in order to obtain optimumdesulfurization. This point may be easily experimentally determined fora particular stream undergoing desulfurization. For example, employingas a feed the residue represented by the overhead designated by thesolid lines of FIG. 5, the best results are obtained by ilashing at thetemperature which gives about a 650 F. separation regardless of theelevated pressure of the flash. This provides not only sufficient totalamount of aromatics in the stream, but a ratio of about 72 percentsaturates by volume to about 26 percent aromatics in the overhead. Aswill be hereinafter demonstrated, the employment of an excessive amountof aromatics in the desulfurization reaction can be as detrimental tosulfur removal as employing too little aromatics.

The dashed lines of FIG. 4 designate another possible overhead saturatesand aromatics distribution in which aromatics begin to predominate in a550 F. overhead fraction. However, since a 550 F. flash includes all thelighter material, the total overhead will still predominate in saturatesover aromatics.

Referring again to FIG. 1, a predetermined amount of lliquid is flashedand removed by means of line 92. Depending upon the nature of thefeedstock employed and the conditions utilized in the desulfurizationreaction, the flashed material in line 92 may contain, for example,between about 5 and about 60 percent by Weight of the liquid eflluentfrom the desulfurization reactor 60, preferably between about l0 andabout 3'5 percent by weight. Suitable flash temperatures include, forexample, between about 500 and about 800 F., preferably between about600" and about 700 F. (These temperatures refer to atmospheric pressureand of course will be different at the process pressure). An especiallypreferred flash point for the interstage flash is 650 F. The mostdesirable amount of liquid to be flashed or otherwise separated from thedesulfurization effluent stream may be easily determined experimentally.The flashed material in line 92 is passed to a high pressure flashchamber unit 94 wherein hydrogen, hydrogen sulfide and light hydrocarbongases are separated from a liquid hydrocarbon fraction. The gases arewithdrawn by means of line 96 and are'subjected to purification andseparation, including various scrubbing operations and the like inrecycle gas recovery unit 97. Subsequently, all of the hydrogen sulfidethat it produced in reactor 60 is removed by means of line 93. Hydrogennow free from hydrogen sulfide and light hydrocarbon gases is recoveredfrom unit 97 and is passed by means of line 99 to recycle gas compressor101 and recycled for utilization in the process by means of line 103.The non-flashed liquid fraction is discharged from flash unit 94 bymeans of line 98.

The bottoms fraction from the ash unit 90 is discharged by means of line100 and is admixed with makeup hydrogen, which is provided by means ofline 102, valve 105 and line 107. In addition, recycle hydrogen may beadded to the make-up hydrogen in line 107 from line 103 by means of line109, valve 115 and line 113. If desired, make-up hydrogen may be passedfrom line 103 directly to line 108 by appropriate valving (not shown).The combined stream may be charged to a furnace 106 in order to raisethe temperature of this stream if desired. However, furnace 106 isoptional, as the stream 104 may be already at the desireddesulfurization temperature for introduction by means of line 108 to thesecond hydrodesulfurization reactor 110.

The temperatures and pressures employed in reactor 110 may be the sameas those described for the hydrodesulfurization reactor 60. Likewise,the desulfun'zation catalyst which is employed in reactor 110 may beidentical to that described previously for reactor 60. It is noted atthis point that the hydrodesulfurization catalyst is even more activefor removal of nickel and vanadium than it is for removal of sulfur.Most of these metals will be removed in the rst hydrodesulfurizationreactor 60. The heaviest laydown of such metals is at the inlet toreactor 60. The second desulfurization reactor 110 will act as a metalsclean-up stage, and the catalyst therein will not collect as much metalsas does the catalyst in reactor 60. Hence, the lirst stage catalyst willremove most of the metals and will become deactivated by metals muchfaster than the second stage catalyst.

The desulfurized ellluent from reactor 110 is discharged by means ofline 114 and is passed to a flash unit 116 for removal of light gasesincluding hydrogen, hydrogen sulfide and light hydrocarbons. Thisgaseous stream is sent by means of line 118 to high pressure flash unit94. Meanwhile, a bottoms fraction including the asphaltic product streamof the present invention is passed by means of line 120 to adistillation column 122. A sulfur containing stream comprising sour gasand sour water is removed from column 122 by means of line 124. Thisstream is passed to a gas treatment plant (by a means not shown) torecover sulfur therefrom. A naphtha fraction is withdrawn from thecolumn 122 by means of line 132. This naphtha stream may be employed asa wash liquid for the separation of the light hydrocarbons from thehydrogen in line 96 (by a means not shown). A furnace oil or heavierfraction may be withdrawn through line 133 and be employed in a mannerhereinafter described to provide additional aromatics to the secondstage desulfurization reactor 110. A product stream 134 is dischargedfrom the distillation column 122. This desulfurized oil stream containsasphaltenes and resins, and is especially useful, without furtherblending, as a fuel oil, particularly since less than one percent byweight sulfur is contained therein. Thus, this heavy, asphaltic fuel oilcontains, for example, between about 0.3 and about 0.5 percent by weightsulfur or less, which is well within the requirements of even thestrictest ordinances for sulfur content of heavy fuel oils.

A modification of the process shown in FIG. 1 is illustrated in FIG. 6,which is a simplified schematic diagram.

Referring now to FIG. 6, a reduced crude oil is introduced |by means ofline 220 to an atmospheric distillation unit 222 wherein a lightasphalt-free distillate fraction having a 630-650 F. end point (E.P.) iswithdrawn by means of line 224, while a heavy asphalt-containing 630650F.| fraction containing 4 percent sulfur is discharged by means of line226 from the distillation column 222. The asphalt-containing bottomsfraction 226 is passed to a vacuum distillation unit 228 whereinadditional light oil is discharged by means of line 230 and is admixedwith the lighter fraction in line 224 and introduced along with hydrogenfrom line 233 into a hydrodesulfurization zone 232 by means of the line234. Zone 232 may be operated with a conventional gas oildesulfurization catalyst but at a lower temperature and hydrogenpressure (e.g., below 1000 p.s.i.) than are employed for thedesulfurization of an asphaltic oil. The asphalt-free distillate iseasily completely desulfurized in the zone 232 and is discharged bymeans of line 236 and passed to distillation unit 238 from which anoverhead fraction containing hydrogen, hydrogen sulfide and light gasesis removed by line 240 and processed as previously described to recoverhydrogen and light hydrocarbons.

An aromatic-rich, furnace oil and higher fraction is discharged from thedistillation unit 238 by means of line 244. Meanwhile, anasphalt-containing oil is withdrawn from the vacuum distillation unit228 by means of a line 246. This stream may have, for example, aninitial boiling point of about 1000 F. and contain about 5.5 percent byweight sulfur. The stream 246 is passed through a blending zone 250wherein the asphalt-containing stream is admixed with a controlledportion of the aromatic-rich fraction from the line 244 in order to0btain the desired viscosity and solvency for the asphaltenes and resinscontained in the stream. Then hydrogen is added through line 248. Theheaviest product from distillation unit 242 can be blended with theproduct 268, if desired.

Next, the asphaltic fraction containing solubilized resins andasphaltenes is passed by means of line 252 to a irsthydrodesulfurization zone 254 and subjected to desulfurization under theconditions previously described in regard to the reactor 60 of FIG. 1.The eflluent from zone 254 has a reduced sulfur content and is passed bymeans of line 256 to an interstage ash unit 258 wherein hydrogen,hydrogen sulfide, light hydrocarbon gases, and a controlled portion ofaromatics and saturates is discharged by means of line 260. Aspreviously discussed, a ash point is selected so as to optimize theamount of aromatic solvent available for solubilizing the asphaltenesand resins in the feed to the second hydrodesulfurization zone. Aneluent stream 262 is discharged from the ash unit 258 and is admixedwith hydrogen which is introduced by means of line 264. The combinedhydrogen-oil stream is passed to a second hydrodesulfurization unit 266wherein the sulfur content of the asphaltic oil is reduced to below onepercent by Weight. The heavy oil product stream is discharged from thesecond hydrodesulfurization unit 266, which unit is operated in themanner described for unit 110 in FIG. l, and is withdrawn by means ofthe line 268 and treated as previously described for separation ofhydrogen sulfide, light gases and the like.

Thus, the system of FIG. 6 provides a parallel mode of operation whereinan initially-separated lighter portion of the crude is desulfurized andis utilized to provide the desired viscosity and solvency fordesulfurization of the heavy, asphaltic portion of the crude oil.

Another modification of the FIG. 1 process is illustrated in FIG. 7.Referring now to FIG. 7, an asphaltic oil is introduced by means of line320 to a distillation unit 322 for separation into an aromatic-poorfraction on which is withdrawn from unit 322 by means of line 324 and anaromatic-rich fraction containing 4 percent sulfur. An aromatic-richasphaltic oil is discharged from unit 322 by means of a line 326. Forexample, distillation unit 322 may be operated to provide an asphalticfraction having an initial boiling point of about 650 F.

The asphaltic stream in line 326 is admixed with hydrogenv which isintroduced by means of line 328 and the combined stream is pased bymeans of line 330 into a 1 of 800 F.l is discharged means of the line340.

Referring momentarily to FIG. 5, it is seen that at an 800 F. ash pointthere is a relatively high aromatic to saturate ratio. However, thetotal aromatic content of the oil at this point may be less thandesired. Accordingly, a controlled amount of an aromatic-rich fractionis introduced into the stream in line 340 by means of line 342 in orderto provide the resins andasphaltenes with a proper degree of solvency.The combined stream is then admixed with hydrogen, which is introducedby means of line 344 and introduced into hydrodesulfurization reactor346 where the sulfur levelV of the asphaltic fuel oil is reduced tobelow one percent by weight.

The etlluent from zone 346 is removed by means of line 348 and isintroduced into distillation unit 350 where an aromatic-rich fraction isseparated and recovered by means of line 352. A controlled portion ofthe material in stream 352 is recycled by means of line 342 foradrnixture with the asphaltic stream in line 340 as previouslydescribed. The light gases are discharged from distillation unit 350 bymeans of line 354, while a substantially sulfur-free, asphaltic, heavyfuel oil is recovered from line 356. Substantially all ofthe'asphaltenes and resins fed to distillation unit 350 are recovered inline 356 with the asphaltic fuel oil. The recycle stream 342 is devoidof asphaltenes. The asphaltenes are not recycled to the rsthydrodesulfurization zone, since they would deactivate the catalystprematurely. They are not recycled to the second hydrodesulfurizationzone, since they have already been desulfurized, such recycle Wouldserve no useful purpose.

In the foregoing manner depicted in FIG. 7, a light, desulfurized,aromatics-rich solvent for the asphaltic material is obtained from theproduct stream.

Still another modification of the present invention is shown in FIG. 8,wherein an sphaltic feed stream is introduced Iby means of a line 420 toa distillation unit 422 to reduce the feed and prepare ahydrodesulfurization feed for passage through line 426. The asphalticfraction is discharged by means of the line 426 from um't 422 and isadmixed with a controlled portion of an aromatic-rich stream which isintroduced by means of line 428. The stream inline 428 can be anaromatic-rich fraction boiling within the range of between about 400 andabout 1050 F., preferably between about 650 F. and about 900 F. Thecombined stream is admixed with hydrogen, which is introduced by meansof line 430, and a stream having a boiling point of about 650 R+ andcontaining about 4 percent by weight sulfur is introduced by means ofline 432 into hydrodesulfurization zone 434.

This unit may be operated at a temperature of 690- 790 or 800 F. Aneluent stream having a sulfur content of about one percent by weightsulfur is introduced by means of line 436 into flash unit 438. A lightoil and gas fraction containing substantially all of the hydrogen suldeproduced is flashed from the unit 438 and discharged by means of theline 440, which stream may have, for example, a 650 F. EP.

An asphaltic stream having a boiling point of, for example, 650 R+ isadmixed with hydrogen which is introduced by means of line 442 andintroduced by means of line 442 and introduced by means of line 444 intosecond hydrodesulfurization zone 446, which may be also operated atabout 690 to 790 or 800 F. The eiiiuent from the second desulfurizationzone 446 has a sulfur content of less than one percent and is passed bymeans of line 448 to distillation unit 450. An aromatics-rich fractionis withdrawn from unit 450 by means of line 452 and is recycled by meansof line 428 for admixture to the asphaltic oil feedstock to the firstdesulfurization zone 434. Hydrogen sulfide and light gases are withdrawnfrom the distillation unit 450 by means of line 454, While a low sulfurasphaltic fuel oil is recovered by means of line 456. Excess from stream452vnot recycled can be from the flash unit 336 by blended with productin line 456 to reduce the sulfur content of the product.

Thus, the arrangement of FIG. 8 utilizes a low sulfur aromatic-richproduct stream for solubilizing the asphaltenes and resins in a heavydesulfurization feedstock having a relatively high I.B.P.

A modification of the system of FIG. 8 is shown in FIG. 9.

The process of FIG. 8 is similar to FIG. 9, however, as shown in FIG. 9,the eiiiuent from flash unit 438 that is withdrawn by means of line 440is passed to a flash unit which is provided with cooling coils. Hydrogensulfide and light gases are removed in this lower temperature flash,which gases are withdrawn by means of line 443. The remaining heaviereffluent is withdrawn from the flash unit 441 by means of the line 445and is passed by means of pump 447 for admixture with stream 444 forintroduction into the second stage desulfurization unit 446.

The mode of operation of FIG. 9 permits the employment of a iiashtemperature for unit 438, which temperature may be the same as thatemployed in the hydrodesulfurization units 434 and 446. At the same timethe lower temperature flash unit 441 which is provided with coolingcoils permits the separation of hydrogen sulfide and light gases and thereintroduction of a material having the optimum aromatics content andinitial boiling int. poThus, for example, if it were determined that theoptimum interstage ash temperature corresponded to 650 F. at atmosphericpressure and the desulfurization units 434 and 446 are being operated atabout 700 F., the flash unit 438 may also be operated at 700 F. However,the lower temperature flash unit 441 is operated at 650 F. and thuspermits the return by means of the 650 R+ material by means of line 449.

Thus, the modiiication of FIG. 9 avoids the need for reducing thetemperature of the stream in line 436 and. the reheating of stream 444.

Referring now to FIG. 10, an asphaltic, sulfur-containing, hydrocarbonoil is introduced by means of line 520 to distillation unit 522 forseparation of the feed into light gases, which are withdrawn by means ofline 524, and an aromatic-rich fraction, which is discharged by line526, which is to be employed in a manner hereinafter described. Thisaromatic-rich fraction may have a 650 F. E.P.

An asphaltic bottoms fraction having an initial boiling point of, forexample, about 650 F. is discharged by means of line 528 and is admixedwith hydrogen from line 530 prior to introduction into iirstdesulfurization zone 532. The desulfurized eiiiuent from zone 538 isintroduced by means of line 534 into flash unit 536. As before, theoptimum flash point has been previously determined and a light oilfraction containing saturates and aromatics is ashed off along withlight gases and hydrogen sulfide by means of line 538.

The asphaltic oil is passed from the flash unit 536 and mixed withhydrogen from line 540 and introduced by means of line 542 to the secondstage desulfurization zone 546. In addition, the asphaltic feed to thezone 546 is admixed with the aromatic-rich stream 526. Thus, thedistillation operation unit 522 is conducted under conditions so thatthe stream 52'6 has a selected boiling range and aromatics content whichprovides maximum solvation for the asphaltenes present in the feed tothe second desulfurization zone 546. The effluent from zone 546 isdischarged by means of the line 548 and is passed t to distillation unit550 for separation of the asphaltic 15 tion for the same total degree ofdesulfurization as a function of time. As illustrated in FIG. 11, therelative inefficiency of operating a single stage hydrodesulfurizationunit down to 0.5 percent sulfur is demonstrated. On the other hand, whenthe first stage is operated down In addition, it is seen from Table I,that the aromatic content of the feed has gone from 55.45 weight percentto 60.45 weight percent after the first stage flashing, and finally upto 61.91 weight perecnt aromatics in the product. At the same time theweight ratio of aromatics to to a 1 percent sulfur and the second stage1s operated resins plus asphaltenes increases from about 2 to to from al percent level down to 0.5 percent sulfur, the about 4 to 1. Thus,Table I clearly illustrates the criticaldesulfurization constant ismaintained above that 0f the ity of providing suicient aromatics 1n theasphaltic single stage process for each stage of the two-stage process.stream in order to solubilize the resins and asphaltenes In addition, ithas been found that the average twoand to deagglomerate any asphalteneaggregates so as stage catalyst aging rate is about two units per day asto permit desulfurization thereof. Likewise, this Table compared with asingle stage aging rate of about 3.5 shows the criticality of avoidingan excess of liquid inunits per day. Catalyst aging is evidenced by theamount cluding aromatics, especially saturates, beyond what is oftemperature increase required to produce a constant required tocontribute a solubilizing effect, since such an desulfurization, asmetals and code increasingly coat the excess of low-sulfur liquid willonly tend to disperse and catalyst surface with time. dilute thesulfur-containing resins, resins and asphaltenes The following examplesare presented to further illuS- and diminish their chance of contactwith the catalyst. trate the invention. It should be further noted thatthe aromatic content of EXAMPLE 1 the feed to the second stage is richerin aromatics, i.e., 60.45 weight percent than is the feed to the rststage, bAn pghaltcntammg reud rd 011 contalmmg i.e., 55.45 weightpercent. This is due, in part to the 650 Ouctl Welghtererlu .ur .an ymgen ar.e.mtro` F. interstage flashing which removes saturates in muchllc; 1 migo! a lybo es unzaitlondfone ontammg a greater proportionrelative to the aromatics present in mc'lfco alt'mpy enum (,a 5Std lposffmt? non' the flashed, light oil stream. In each stage, the weight cra?.mg. aunm iuppotrt e t y ro fes) ulzsboizo; ratio of aromatics to resinsplus asphaltenes to accomga 10g 1s geg uc e atfenpera res lf) 230311 dplish solvation should be at least 1 to 1 and is preferably th' an t. yroglnlar la tpesllreo h d tp's'tlg" an 1.5 or 2 to l, and can be 4 or 5to 1. The aromatics can e res mg as? a c maoena 1S as e a a empera' bepresent in the feed, can be introduced by recycle or ture correspondingto 650 F. at one atmosphere so as can be produced in Situtotoptlnhlzethe atmntsforgmancs nltisaftrattiesnptls; It is furtherinteresting to note that Table I reveals that .en irtlhd e e geg' the hhlns a culfac o nt at the saturates, which are iiashed off, are the mosthighly Lsfigbourtavlvlgr m. hte rsc tglullfu asn si ilrtri elddesulfurized fraction and therefore have the least need .nto comgv'ilgod gulf r9 afan roi re th for passage through the second stage ofdesulfurization. 1 a. se. y r e u lz l Z e 2V e e, e' The followingexample illustrates the refractory nature sulfurization is alsoconducted at about 650 F. to 820 F. While em lo in the same Catal stthat is em lo ed in of an asphaltic feedstock when it 1s attempted toremove p y g y p y more than one percent sulfur without employing theprocthe iirst stage.

ess of the present invention. A heavy fuel oil 1s obtained having asulfur content EXAMPLE 2 of 0.58 weight percent sulfur. The distributionof sulfur in each of the various fractions of the oil undergoing 40 A 22percent reduced Kuwait crude containing an desulfurization is set forthin Table I, below: asphalt fraction and 5.43 percent by weight sulfur isTABLE I Feed to first HDS Feed to second HDS zone zone Fuel oil productSulfur in Sulfur in Sulfur in Fraction fraction Fraction fractionFraction fraction (percent (percent (percent (percent (percent (percentby Wt.) by Wt.) by wt.) by Wt.) by wt.) by wt.)

17. 98 3. 42 22. 24 o. so 22. 34 0. 49 55. 5.04 60. 45 1. i2 6i. 9i o.56 16. 73 5. 59 13. 76 2. 37 12. 72 1. 5e asphaltenes 9. s4 6.99 3.55 4.95 3.03 3.13

As seen from Table I, it is apparent that in an assubjected todesulfurization. The initial boiling point of phaltic feed containing atotal of about 4.09 weight perthe crude is 556 and the boiling rangeextends to 1400 cent sulfur, the sulfur content of such feed isrelatively R+. After the sulfur content is reduced to 4.77, the

evenly distributed between the saturates, aromatics, resins andasphaltenes. However, after the feed is passed through the irstdesulfurization zone and the 650 F. E P. fraction is removed, i.e., at1.09 weight percent total sulfur, the saturates and aromatics have lostsulfur to the greatest extent, viz, they are down to 0.80 and 1.12weight percent sulfur respectively, while the resins and asphalteneshave lost sulfur to the least extent, viz, 2.37 and 4.95 percent byweight sulfur, respectively. Finally,

Sulfur, percent by wt Boiling range, F Desulfimzation, percent APII.B.P. is 514 and the boiling range extends to 1400 As desulfurizationcontinues, and the sulfur content is reduced to 1.41, the I.B.P. is 509with the boiling range extending to 1400 R+. However, when the sulfurcontent is reduced to 0.83, the I.B.P. drops F. to 466 F. with theboiling range extending to 1400 R+.

The results of this run are shown in Table II, below:

TABLE II Reduction in sulfur content dur-lng Feedstock desulfurization5. 43 4.77 1.4i asc-1,400+ 514-1,4oo+ 50e-1,400+ 466-1, 12.2 74.0

The foregoing data shows that the sulfur content of the feed becomesvery refractory at about 74 percent desulfurization. At 12 percentsulfur removal the I.B.P. is

reduced to 415 F., while at 74 percent sulfur removal the I.B.P. is onlyreduced to 509. However, in order to obtain 85 percent sulfur removal,the I.B.P. is reduced all the Way to 466 F. Thus, in single stageoperation, hydrocrackng begins to predominate over hydrodesulfurization,i.e., carbon-carbon bonds are becoming severed, rather thancarbon-sulfur bonds, beyond 74 percent sulfur removal.

The following Table III, below, shows the composition of the feed duringthe course of the desnlfurization reaction:

TABLE III Feedstock (percent bg Composition change durln wt.desnlfurization (percent by wt.

Desulurlzation. -n 12. 2 74 85 Baturates 11 16. 7 26. 1 33. 2 Aromatics39 41. 2 56. 6 49. 9 esins 32 25. 7 15. 1 15. 3 Asphalt'enes 18 16. 4 2.2 1. 6

'I'he foregoing data show that as the depth of sulfur removal increases,resins and asphaltenes diminish and are converted to saturates andaromatics.

The following Table IV gives a molecular weight comparison of thearomatics, saturates and residuals during the course of desnlfurization.

TABLE IV Molecular weight change Feedstock during desulfurizatiouDesulfurization, percent 12. 2 74 85 saturates, M.W 430. 400 410Aromatics M.W 490. 0 530 400 Total residuals, M.W--- 1, 080 590.0 490420 The data in Table IV show that as the depth of desulfurizationincreases, the molecular weight of the total residuals decrease, but notsignificantly below the molecular weight of the initially formedsaturates and aromatics. This molecular weight pattern furtherillustrates that during the breaking of carbon-sulfur bonds in theresins and asphaltenes, hydrocarbon fragments are produced which areabout in the same molecular weight range as are the feed saturates andaromatics.

, EXAMPLE 3 lb. oil Ib. oil lb. sulfur hr.1b. catalyst This reactionrate constant can also be expressed as For comparative purposes, adesulfurized furnace oil having a boiling point in the range of 400 to650 F. and containing 0.07 weight percent sulfur is added to the residueof the 650 F. dash operation. The furnace oil is comprised of aboutone-half saturates and one-half aromatics. Upon passing this asphalticfeed through a second stage desulfurization, the desnlfurizationreaction rate drops to 75. Thus, excessive dilution of the feed to thesecond stage actually reduces the desulfurization rcaction rate, eventhough aromatics are being added.

For further comparison, 30 percent by Weight of a light portion of thesecond desnlfurization stage feed is removed making the feed equivalentto about the residue of an 800 F. ilash. In this instance, the reactionrate for desnlfurization in the second stage falls to 40 therebyillustrating the effect of inadequate solvation for the resins andasphaltenes. It is clearly evident that the amount of saturates plusaromatics which accompanies the resins and asphaltenes into the secondstage has a distinct effect upon the second stage reaction rate, andthat too much diluent can have a detrimental eect upon desulfurizationreaction rate, just as does too little aromatic solvent.

EXAMPLE 4 In order to determine the effect of adequate, as contrasted toexcessive solubilization of an asphalt-containing feed, a residual,high-boiling feed having au initial boiling point of about 800 F. ischarged to a hydrodesulfurization process, and 76.2 percent by weightdesulfurization is accomplished. Next, a second sample of the feed isdiluted with 30 volume percent of a lower boiling gas oil which had beenpreviously desulfurized to the extent of to 95 percent by weight. Theaddition of the gas oil, which comprises a high proportion of aromatics,increases the desnlfurization to 80.3 percent by weight.

For comparative purposes, a further sample of the feed is diluted with40 percent by volume of a gas oil and is subjected to desulfurization asbefore. In this instance, there is a loss in desnlfurization activityand the desulfurization drops to 76.3 percent by Weight. The developmentof 64 percent by volume of gas oil reduces the degree of desnlfurizationeven lower to 69.4 percent by weight. The results of these tests areillustrated in Table V below:

TABLE V Eect of gas oil dilution Gas oil, percent by vol 0 30. 0 40. 064. 0 Desulunzatiou, percent by Wt 76. 2 80.3 76. 3 69. 4 Vanadiumremoval, percent by wt 83. 5 78.6 74. 6 77. 8

EXAMPLE 5 A test is performed utilizing in the iirst and second stages aone-thirty-second inch nickel-cobalt-molybdenum on alumina catalyst. Inthe iirst stage the catalyst exhibits a six month life with astart-of-run temperature of 690 F. and an end-of-run temperature ofabout 790 F. in hydrodesulfurizing a Kuwait reduced crude from 4 to 1weight percent sulfur at a LHSV of about 0.8. The catalyst life in thesecond stage is even longer. The one weight percent sulfur etlluent fromthe iirst stage is flashed to remove 650 F. end point material based onatmospheric pressure and is charged to the second stage with hydrogen toreduce the sulfur level to 0.5 Weight percent. The second stagestart-of-run temperature is 690 F. and the temperature at days is only763 F. The test can continue until the temperature is 790 F. Therefore,the first and second stages of this invention can operate for 3, 4, 5, 6or even 7, 8 or l2 months at a LHSV generally ranging between 0.1 and 10or preferably between 0.3 and 1.25. The life of the second stagecatalyst is longer than the life of the first stage catalyst.

Although the invention has been described in considerable detail withparticular reference to certain preferred embodiments thereof,variations and modifications can be effected within the spirit and scopeof the invention as described hereinbefore, and as defined in theappended claims.

We claim:

1. A process for the hydrodesulfurization at a hydrogen partial pressurebetween 1,000 and 5,000 p.s.i. in a twozone reaction of a heavyasphaltic feed oil to produce a heavy asphaltic fuel oil productcontaining propaneand pentane-insoluble asphaltenes andpropane-insoluble resins but pentane-soluble resins, comprising passinga sulfur-containing asphaltic hydrocarbon oil feed downfiow withhydrogen through a first zone containing at least one bed ofhydrodesulfurization catalyst particles y2() to 1/40 inch in diameter ata temperature between 680 and 800 F., the temperature being increasedduring the reaction to compensate for loss of hydrodesulfurizationreaction rate due to catalyst aging, said catalyst particles comprisinga Group VI and a Group VIII metal on alumina containing less than onepercent silica, withdrawing a first effluent stream from said firsthydrodesulfurization zone having a reduced sulfur content, said firsteliiuent stream comprising a light gas fraction including hydrogensulfide, a light oil fraction containing saturates and aromatics, and ahigher-boiling asphaltic fraction containing asphaltenes and resins,separating said light gas fraction and a portion of said light oilfraction from said first efliuent stream, said light oil fractionboiling between about 500 and 800 F. when measured at atmosphericpressure, passing said higher-boiling asphaltic fraction containingasphaltenes and resins and the remaining portion of said light oilfraction with additional hydrogen downflow through the second zonecontaining at least one bed of hydrodesulfurization catalyst particleslyo to JA@ inch in diameter at a temperature between 680 and 800 F., thetemperature being increased during the reaction to compensate for lossof hydrodesulfurization reaction rate due to catalyst aging, said secondZone catalyst comprising a Group VI and a Group VIII metal on aluminacontaining less than one percent silica, operating the catalyst in saidfirst and second zones for at least three months at a liquid hourlyspace velocity between 0.1 and 10, maintaining the average hydrogenpartial pressure in each of said zones sufficiently high that theaverage hydrodesulfur-ization reaction rate constant is improved by saidtwo zones of operation and accomplishing a degree of desulfurizationwith less catalyst as compared with a single zone of operationaccomplishing the same degree of desulfurization, withdrawing a secondefiiuent stream from said second hydrodesulfur-ization zone comprising alow sulfur heavy asphaltic hydrocarbon fuel oil, and recovering saidproduct having less than 1 weight percent sulfur, of said product notless than 50 20 percent by weight having a boiling point greater thanthe initial boiling point of the feedstock and not more than 20 weightpercent having a boiling point lower than 375 F., said productcontaining saturates, aromatics, resins and asphaltenes.

2. The process of claim 1 wherein said average hydrogen partial pressureis between 1400 and 2200 p.s.i.

3. The process of claim 1 wherein said average hydrogen partial pressureis greater in the second zone than in the first zone.

4. The process of claim 1 wherein said hydrogen partial pressure in thesecond zone is between about 1400 and 2200 p.s.i. I Y

5. The process of claim 1 wherein the second efiiuent hydrocarboncontains less than one percent sulfur.

6. The process of claim 1 wherein said second zone reduces thehydrocarbon stream sulfur'content from at least one percent to less than0.5 percent.

7. The process of claim 1 wherein resins and asphaltenes comprise 5 to30 percent or more of the feed.

8. The process of claim 1 wherein at least 75 percent sulfur is removedfrom the feed in the first zone.

9. The process of claim 1 wherein the second efiiuent stream contains atleast percent by weight of material boiling above the initial boilingpoint of the feed.

10. The process of claim 1 wherein not more than 10 percent of thesecond efiiuent boils below 650 F.

11. The process of claim 1 wherein the catalyst contains nickel, cobaltand molybdenum.

12. The process of claim 1 wherein the catalyst age in both zones is atleast four months.

13. The process of claim 1 wherein the catalyst age in both zones is atleast five months.

14. The process of claim 1 wherein the liquid hourly space velocity isbetween 0.3 and 1.25.

15. The process of claim 1 wherein the liquid hydrocarbon efiiuent fromthe second zone boiling below 375 F. does not exceed l0 weight percent.

References Cited UNITED STATES PATENTS 3,617,525 11/ 1971 Moritz et al.208-211 3,418,234 12/ 1968 Chervenak et al. 208-210 3,530,062 9/ 1970Gatsis 208-89 3,481,860 12/1969 Borst, Jr. 208-213 3,471,398 10/ 1969Borst, Jr. 208-209 DELBERT E. GANTZ, Primary Examiner G. J. CRASANAKIS,Assistant Examiner

